Experimental results are presented which suggest that parameters based on the J-integral and the crack opening tip displacement δ are viable characterizations of crack initiation and stable crack growth. Observations based on some theoretical studies and finite-element investigations of the extending crack revealed that J and δ when appropriately employed do indeed characterize the near-field deformation. In particular, the analytical and experimental studies show that crack initiation is characterizable by the critical value of J or δ, and stable crack growth is characterizable in terms of the J or δ resistance curves. The crack opening angle, dδ/da, appears to be relatively constant over a significant range of crack growth. Thus, appropriate measures of the material toughness associated with initiation are JIc and δIc, and measures of material toughness associated with stable crack growth are given by the dimensionless parameters TJ [= (E/σo2)(dJ/da)] and Tδ [= (E/σo)(dδ/(da)]. The two-parameter characterization of fracture behavior by JIc and TJ or δIc and Tδ is analogous to the characterization of deformation behavior by the yield stress and strain hardening exponent.
A JIc test procedure using a single deeply cracked specimen is proposed. The crack extension is measured by partially unloading the specimen to determine the elastic compliance. JIc tests were made using ASTM A469 steel. Compact specimens from 1/2T to 5T were tested. No size effect was found. Results from two independent laboratories are presented and are in agreement. The errors due to simple formulation of JI calculation, periodic partial unloading, and simplified analysis for the extension of deep cracks in compact specimens are explored. The measurement point of crack extension for establishing JIc is discussed. The results indicate that a practical and effective single specimen test procedure has been developed.
A test program was conducted to determine the effects of specimen thickness variations, side grooves, and crack length variations on the deformation and ductile fracture of A533-B, Cl-1 steel at 93°C. The crack extensions were estimated using the correlation between elastic compliance and crack length. Crack extensions were also estimated using a correlation among crack-opening displacements, load line displacements (δ - VL), and crack length. The inferred estimates of crack extension were supplemented by some measurements on heat-tinted fracture surfaces. The results suggest that the observation of thickness or side-groove effects on crack-extension resistance curves is dependent on the method of measuring crack extension. The compliance correlation method was less sensitive to crack extension and showed a classical thickness effect: increased crack growth resistance with decreasing thickness, and decreased resistance with the use of side grooves. The δ - VL correlation method was more sensitive to crack extension and showed no effect of thickness or of side grooves on crack growth resistance. The presence of side grooves promoted flat fracture and suppressed shear lips. Specimens without side grooves developed large shear lips.
Accurate estimates of valid plane-strain fracture toughness, KIc, for low-alloy steels in the ductile-to-brittle transition temperature range may be made by using JIc-valid specimens and accounting for a size effect evident for cleavage fracture. The size effect is explained using a “weakest link” theory that predicts variance in test results using constant-size specimens and decreasing average results when the specimen size is increased. Cleavage fracture occurs when the maximum tensile stress near the crack tip equals or exceeds the cleavage stress, σf*, over a microstructurally significant distance, the process-zone diameter. Variation in specimen toughness arises due to variations in the microstructure within the process zone; the weakest feature in the process zone causes catastrophic cleavage fracture of the specimen. Our work indicates that the size effect on JIc is represented by J1∕J2=(B2∕B1)1∕m where m is the Weibull modulus, Bi is the specimen thickness for specimen i, and Ji is the mean JIc for several specimens. Experimental evidence supporting the theory is shown.
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